Circular loop

[de Source]

hi..
i want to know the the syntax of for loop that can circulate betweent "0" to "63" with step "8" that mean if value of "i" become 60 it will not move to 68 it should be go back to zero after 63 and the value of "i" should become"5"
Thanks..

[de Source]

let me be more specific
i have an array of 64 values i have to pass it through PC1 table (DES).

57  49  41  33  25  17  9
1   58  50  42  34  26  18
10  2   59  51  43  35  27
19  11  3   60  52  44  36
63  55  47  39  31  23  15
7   62  54  46  38  30  22
14  6   61  53  45  37  29
21  13  5   28  20  12  4

in a manner that arr[57] come first then arr[49] and so on
what could be the best procedure for this
thanks

[Momerath]

int i = (i + 8) % 64;